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United States Patent |
6,084,701
|
Coleman
|
July 4, 2000
|
Electrochromic tin oxide
Abstract
Tin oxide doped with an electrochromically effective amount of a metal,
e.g. antimony or niobium, which provides a color change when the doped tin
oxide is exposed to an electrochemical potential in the presence of mobile
ions. Particles of electrochromic doped tin oxide, e.g. coated on a white
or pastel pigment substrate, have a contrast ratio greater than 1.2, where
contrast ratio is a measure of electrochromic functionality of a material
and is the ratio of reflectance of the material in an oxidized state to
the color of the material in a reduced state and where color is a
photodiode measurement of the value of light reflected off the oxidized or
reduced material from a constant source of light shining on the material.
Such doped tin oxide-containing particles are useful as electrochromic
material in display devices.
Inventors:
|
Coleman; James P. (Maryland Heights, MO)
|
Assignee:
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Monsanto Company (St. Louis, MO)
|
Appl. No.:
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256829 |
Filed:
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February 24, 1999 |
Current U.S. Class: |
359/273; 359/265 |
Intern'l Class: |
G02F 001/153; G02F 001/15 |
Field of Search: |
359/265-274
|
References Cited
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| |
Other References
Masumi, Response-Improved Electrochromic Display Based on Organic
Materials, Proc. of SID 23/4:245-248 (1982).
Nomura et al., Electrochemical and Electrochromic Properties of Polymer
Complex Films Composed of Polytetramethyleneviologen and
Poly-[p-styrenesulfonic Acid] Containing a Conductive Powder, J. Macromol.
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(1984).
|
Primary Examiner: Epps; Georgia
Assistant Examiner: Spector; David N.
Attorney, Agent or Firm: Howell & Haferkamp, L.C.
Parent Case Text
This is a divisional of application Ser. No. 08/773,210; filed on Dec. 23,
1996 now U.S. Pat. No. 5,876,634 which claims priority to Provisional
application Ser. No. 60/009,226 filed Dec. 26, 1995.
Claims
What is claimed is:
1. An electrochromic device comprising a layer of electrochromic material
in contact with an ion-supplying electrolyte layer and conductors for
applying an electrical potential across an interface between said layer of
electrochromic material and said ion-supplying electrolyte layer, where in
said electrochromic material comprises doped tin oxide particles.
2. A device according to claim 1 wherein said electrochromic material
comprises antimony-doped, tin oxide particles on a substrate of or in
admixture with a white or pastel pigment.
3. A method of producing an electrochromic contrast ratio greater than 1.2
by applying an electrochemical potential to doped tin oxide particles in
contact with mobile ions.
4. A device according to claim 1, wherein said electrochromic material
comprises niobium-doped tin oxide particles on a substrate of or in
admixture with a white or pastel pigment.
Description
Disclosed herein are novel electrochromic materials comprising tin
oxide-containing particles having a high contrast and methods of making
and using such materials, e.g., in electrochromic devices.
BACKGROUND OF THE INVENTION
Doped tin oxides are known among metal oxides for their relative
transparency and high electrical conductivity. These properties are
advantageously employed in a variety of electro-optical applications,
e.g., providing transparent conductive coatings on particles or surfaces.
One such application is the fabrication of transparent electrodes on
electrochromic display devices which typically have an electrolyte
material in contact with an electrochromic material so that an
electrochromic effect is generated when an electric potential is applied
across the interface of the two materials. When electrodes are provided on
both sides of the materials, e.g., in sandwich-like structure, the
electrode on at least one side of the display laminate needs to be
relatively transparent to permit observation of the electrochromic effect.
In such devices typical electrochromic materials include tungsten oxide,
Prussian Blue, polyaniline and viologens. Transparent electrodes have been
fabricated by vapor deposition of antimony-doped tin oxide (ATO) coating
on a glass or plastic substrate.
Although doped tin oxides have been employed as transparent conductors in
electrochromic devices, it appears that the possibility that doped tin
oxide might be useful as a practical electrochromic material has not been
discovered. For instance, Orel et al. reported in the Journal of the
Electrochemical Society, Vol. 141, page L127 (1994) that a film of ATO
exhibited a change in light reflectance between a reduced and oxidized
state of less than 5%, which corresponds to a contrast ratio (as defined
hereinbelow) of less than 1.05. Because such a change in color is not
readily discernable to the typical human eye, it has not been recognized
or discovered that doped tin oxides have useful electrochromic properties.
A variety of dopants are used to make conductive metal oxides, some of
which, e.g., fluorine-doped tin oxides are not known to exhibit any useful
electrochromic effect regardless of modification. Similarly, ATO, when
provided in film form, also does not exhibit any useful electrochromic
effect. When select doped tin oxides, e.g., ATO and niobium-doped tin
oxide, are provided in particle form in an electrochromic generating
environment, a surprising electrochromic effect is achieved. Thus, this
invention is directed to the surprising discovery that certain of the
conductive doped tin oxides can be useful high contrast electrochromic
materials and to electrochromic devices employing such electrochromic
doped tin oxides.
SUMMARY OF THE INVENTION
This invention provides novel electrochromic materials comprising doped tin
oxide having a high contrast ratio between different oxidation states.
These high contrast electrochromic tin oxide materials are doped with an
electrochromically-effective amount of a metal that provides a color
change when exposed to ion transfer in an electric field. Preferred
dopants are antimony and niobium.
The invention also provides methods of making such electrochromic tin oxide
materials, for instance in the case of antimony-doped tin oxide materials
by employing higher levels of antimony than commonly used in conductive
tin oxide applications.
This invention also provides methods of advantageously using such
electrochromic tin oxide materials, e.g., in display devices. More
particularly, this invention also provides a method of producing an
electrochromic effect by applying an electrochemical potential to doped
tin oxide in contact with mobile ions.
This invention also provides particulate antimony-doped tin oxide that is
oxidized or reduced to provide a powder resistivity which is at least two
times the powder resistivity of a base antimony tin oxide compound. Such
resistive antimony-doped tin oxide is also uniquely colored as compared to
analogous tin oxides that are highly conductive and transparent.
This invention also provides electrochromic devices comprising such high
contrast, electrochromic tin oxide materials. Such devices are typically
laminate structures comprising a layer of electrochromic material in
contact with an ion-supplying electrolyte layer.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Percentages expressed herein as "%" are mole percent unless indicated
otherwise, e.g. weight percent is expressed as "wt%".
As used herein "powder resistivity" means an electrical resistivity
measured with a two probe apparatus on powder compressed at high pressure
in a confined but defined space. The two probes of the apparatus comprise
metal, e.g. stainless steel, rods about 6.5 mm in diameter and extending
about 15 mm from the center of a plate, e.g. a disk that is 9 mm thick and
5 cm in diameter; the disks are electrically connected to an ohm-meter.
The overall length of the extended rods is slightly longer than the length
of a hole in a non-conducting, e.g. acrylic polymer, cylinder reinforced
by a surrounding metal, e.g. aluminum, sleeve and having a central bore
that is slightly larger than the diameter of the rods. To measure powder
resistivity, the cylinder is mounted on one disc with the rod inserted
into the bore; the open bore is partially filled with particulate material
which is compressed by pressing the second rod into the bore. Resistance
is measured by an ohm-meter when the pressure on the powder is 845
kilograms per square centimeter (which is equivalent to 12,000 psi). At
that pressure the height of compressed particles in the bore (H.sub.p) is
determined by measuring the space between the discs in centimeters with a
micrometer. The cross sectional area of the rod (A.sub.r) is 0.3318 square
centimeters. Powder resistivity (.rho.) is determined by multiplying the
measured resistance by the ratio of cross sectional area (A.sub.r) to
height of compressed particles in the bore (H.sub.p).
As used herein the terms "oxidized" and "reduced" mean changing the number
of electrons associated with a valence state of a material by chemical or
electrochemical means. An oxidized metal oxide is a material that has
fewer electrons than the same metal oxide material in its natural state.
Conversely, a reduced metal oxide is a material that has more electrons
than the same metal oxide material in its natural state. A common chemical
reducing agent is sodium borohydride which is capable of putting electrons
accompanied by ions, e.g. protons or other cations such as sodium,
lithium, etc., into a tin oxide lattice. In a reduced state, e.g. when
saturated with electrons, doped tin oxide has a darker color and lower
conductivity. Interestingly, the conductivity of both reduced and oxidized
doped tin oxide is significantly lower than the conductivity of doped tin
oxide in the natural state. The conductivity of oxidized doped tin oxide
is lower because the number of electron carriers is substantially lower.
The conductivity of reduced doped tin oxide is lower because the material
is so saturated with electrons that electron mobility is impaired. To
achieve an electrochromic effect it is believed that it is necessary to
cause an electrochromic-generating oxidation or reduction at the surface
of the electrochromic material, e.g. typically by applying an electrical
potential of 1.5 volt or less to cause cations and electrons to migrate
into or out of the surface layer of the electrochromic material.
As used herein the term "contrast ratio" (CR) describes the difference in
color of a material in oxidized and reduced states. More particularly,
contrast ratio means the ratio of reflectance of a material in an
electrochemically oxidized state to the reflectance of the material in an
electrochemically reduced state, where reflectance is a photo diode
measurement of the value of light reflected off the oxidized or reduced
material from a constant source of light shining on the material. A
material having a CR=1 would have no electrochromic effect, that is the
light reflected from the material in a reduced state would be immeasurably
different from the light reflected from the material in an oxidized state.
The electrochromic tin oxide materials used in the electrochromic devices
of this invention have a CR of at least 1.2 or higher, say at least 1.4 or
1.6. Preferred electrochromic tin oxide materials of this invention have a
CR of at least 1.8 or higher, say at least 2 or 3. More preferred
electrochromic tin oxide materials of this invention have a CR of at least
4 of higher, say at least 4.5 or 5.
A material is said to be in an oxidized state when it has fewer electrons
than in the reduced state. For instance, in oxides, antimony has two
stable oxidation states with two or zero electrons in the outer shell.
These are denoted as Sb(III) and Sb(V). In the mixed oxide compound ATO,
as naturally formed, the antimony atoms in the metal oxide lattice are at
an oxidation state intermediate between III and V with electrons from the
antimony in a tin 5S electron orbital based conduction band. In ATO the
natural blue grey color is believed to be due to a charge transfer
absorption band.
For electrochromic metal oxides, e.g. ATO, I have now shown that the wide
ranges of color and conductivity in metal oxide powders are possible for a
given ratio of dopant to base metal by changing the oxidation state of the
material, i.e. changing the number of electrons. For instance, in the case
of ATO, the number of delocalized electrons associated with the antimony
can be changed by either chemical or electrochemical oxidation or
reduction.
In particular, in oxidized ATO electrons are removed from the material
effectively transforming the antimony to a species closer to antimony V
which has no free electrons and essentially no color; there are no
delocalized electrons remaining to undergo charge transfer. Conversely, in
reduced ATO there is an increase in delocalized electrons in the metal
oxide, resulting in more charge transfer light absorption and hence more
color. In a natural state, e.g. without exposure to an electrical
potential causing oxidation or reduction, natural ATO has an inherently
translucent blue grey color where the intensity and hue of the color is a
function of the antimony in the mixed metal oxide.
In describing dopants it is useful to use mole ratios of metal components
in the tin oxide material. Unless otherwise indicated, the amount of
dopant in a tin oxide will be expressed as a mole ratio. For example, 6%
ATO describes tin oxide doped with antimony where the number of moles of
antimony as a percent of the combined moles of antimony and tin is 6%.
Factors affecting contrast ratio of a doped tin oxide include the amount of
dopant and the covering power, i.e. ability to absorb light, of optional
adjuvant materials such as pigmented particles mixed with doped tin oxide
particles or providing a substrate for a doped tin oxide coating.
Although doped tin oxide is typically considered transparent or
translucent, especially when applied as a film, particular doped tin oxide
can have perceptible color, perhaps due at least in part to the
interaction of light with the particle. Thus, factors affecting color
within the realm of routine experimentation include particle size, amount
of dopant, crystallite size and dimensional thickness of the doped tin
oxide material. For instance, particles of 10% ATO have what appears to be
a dark gray color. While the dark gray-colored, doped tin oxide can be
used in electrochromic devices, the contrast is often not optimal since
reduced tin oxides generally get darker in color, leaving little room in
the chromatic spectrum for adequate contrast in display images. It has
been discovered, however, that when doped tin oxide is used in combination
with a light-colored pigment substrate, that the lighter color of the
pigment imparts a lighter natural color that provides significantly
greater contrast when the doped tin oxide is reduced and/or oxidized.
Thus, in providing material for use in electrochromic displays, it is
often preferred to provide the doped tin oxide with a light-colored
adjuvant, e.g. a white or pastel colored pigment, that will provide a
light background color visible through a generally transparent or at least
translucent, doped tin oxide. The doped tin oxide and pigment can be
provided as a mixture of particles. It is preferable to provide the doped
tin oxide as a coating on, or in admixture with, a light colored adjuvant
substrate. Useful adjuvant pigments include titanium dioxide (TiO.sub.2),
mica, aluminum borate, silica, barium sulfate and alumina. When doped tin
oxide is used in a mixture with pigment particles, the pigment material is
preferably less electrochemically active in aqueous electrolytes than is
the doped tin oxide. When used with a light colored pigment adjuvant as a
substrate for doped tin oxide, the amount of doped tin oxide in the
coating is not critical so long as the particle is sufficiently
conductive. Unless otherwise indicated, the relative amounts of doped tin
oxide and pigment will be expressed as weight ratio, e.g. a weight ratio
of doped tin oxide to pigment substrate in the range of 1:4 to about 4:1.
Useful pigments have particle size of micrometer (micron) scale, e.g. with
a nominal diameter in the range of about 0.05 to 20 microns and more
typically about 0.2 to 10 microns and more preferably about 1 to 5
microns.
When the adjuvant material is particulate TiO.sub.2 --a commonly used
pigment material with exceptionally high covering power--it has been found
that 6% ATO coated onto TiO.sub.2 in the weight ratio 2:3 ATO/TiO.sub.2
has a CR of 1.2. When the antimony in ATO is increased to about 11 mole
percent, the CR is 1.6. A number of ATO coated TiO.sub.2 pigments which
are commercially available as conductive metal oxide particles having from
1 to about 13% antimony are useful in the displays of this invention. For
instance, a light grey conductive powder comprising 12.25% ATO on 0.2
micron TiO.sub.2 particles in the weight ratio of 23:77 is available from
Mitsubishi Materials Company Ltd. as W-1 conducting particles. Grey
conductive powders comprising 12.3% ATO on 1 to 5 micron TiO.sub.2
particles in the weight ratio of 33:77 are available from E.I. Dupont de
Nemours and Company under the tradenames Zelec.RTM. 1410T and 3410T. Such
commercially available materials have a CR of about 1.6. When the antimony
is increased to 22 mole percent, the CR is surprisingly increased to a
value greater than 2. Thus, one aspect of this invention provides novel
ATO coated TiO.sub.2 particles having a CR greater than 1.6, e.g. at least
about 1.8, more preferably greater than 2.
When other pigments with less covering power than TiO.sub.2 are used, e.g.
ATO on aluminum borate (at a weight ratio of about 0.5), it has been
discovered that doped tin oxide materials with an exceptionally high CR,
e.g. up to about 4-5 can be produced. More particularly it has been
discovered that certain commercially available conductive powders
comprising ATO on pigments such as aluminum borate, barium sulfate, zinc
oxide, silica and mica, are surprisingly electrochromic. In particular, a
grey conductive powder with a surprisingly high contrast ratio, i.e.
greater than 4, is 11.5% ATO on 4 micron aluminum borate particles in the
weight ratio of 54:46 available from Mitsui as Passtran 5210 Type V
conducting particles.
The doped tin oxide materials of this invention can be obtained from
commercial sources or produced by well-known methods with appropriate
adjustment in materials, e.g. dopant level and the amount and nature of
adjuvant pigment, for optimal electrochromic effect. For instance
ATO-coated TiO.sub.2 according to this invention can be prepared by adding
an hydrochloric acid-acidified aqueous solution of antimony chloride
(e.g., the trichloride or the pentachloride) and tin tetrachloride to an
aqueous dispersion of TiO.sub.2 particles, with simultaneous addition of
sodium hydroxide to maintain pH at about 2. This process produces
non-conductive metal hydroxide coated TiO.sub.2 particles which are
converted to conductive, doped tin oxide coated particles when heated to
liberate water, e.g., in the range of 300 to 700.degree. C.
This invention provides particulate antimony-doped tin oxide that is
oxidized or reduced to provide a tin oxide compound that has a powder
resistivity which is at least two times the powder resistivity of a base
antimony tin oxide compound. Particulate antimony-doped tin oxide is
commonly made by thermally treating a mixed antimony and tin compound. For
instance, antimony-doped tin oxide can also be prepared by precipitating
mixed hydroxide particles from a solution of mixed antimony and tin
followed by thermal treatment, preferably at a temperature greater than
350.degree. C., to form particles of base antimony tin oxide compound. The
electrical resistivity of base ATO depends on a number of variables, e.g.
particle size and level of antimony dopant, and can typically range from
0.05 to 10 ohm-cm. With such a wide range of base powder resistivity it is
expected that there can be some overlap with the range of increased
resistivity for oxidized or reduced tin oxide compounds. It has been found
that the powder resistivity is increased more substantially when the doped
tin oxide is reduced rather than oxidized. For instance, as shown in the
following examples, a commercial ATO having a powder resistivity of 0.12
ohm-cm can be reduced to provide a powder resistivity of about 400 ohm-cm
or oxidized to provide a powder resistivity of about 5 ohm-cm. With heat
treatment the powder resistivity can be returned to a value in the range
of the original. When such a base antimony-doped tin oxide is oxidized or
reduced there is provided a resistive, particulate antimony-doped tin
oxide that is characterized as having a powder resistivity which is at
least two times the powder resistivity of said base antimony tin oxide
compound. Preferably such resistive, particulate antimony-doped tin oxide
particles will have a nominal dimension in the range of 0.2 to 10
micrometers. Such resistive particulate doped tin oxide being oxidized or
reduced will also exhibit desirable electrochromic properties.
When the electrochromic doped tin oxide materials of this invention are
used in electrochromic display devices, such materials are typically
disposed in a laminate structure, e.g. a layer of electrochromic material
in contact with a layer of ion-supplying electrolyte. Alternatively,
displays can be fashioned by providing a layer comprising electrochromic
particles in a electrolyte matrix. Commonly, an electrical potential is
applied across the materials by electrodes so that a potential is created
at an interface of electrochromic material and electrolyte. Such electric
potential causes ions, such as protons, lithium ions or sodium ions, to
migrate into or out of the electrochromic material, causing the
electrochromic effect-generating reduction or oxidation. Useful
electrochromic displays can be prepared using the electrochromic doped tin
oxide materials of this invention by following the display fabrication
principles set forth in my earlier U.S. Pat. No. 5,413,739 or other
principles apparent to those skilled in the art.
This invention also provides electrochromic devices useful for displays.
Such devices preferably comprise a layer of electrochromic material in
contact with an ion-supplying electrolyte layer. In a preferred embodiment
the layer of electrochromic material comprises high contrast,
electrochromic, doped tin oxide as disclosed herein and dispersed in a
transparent or translucent polymer matrix in an amount such that the
material is electrically conductive. The polymer of the matrix can
comprise any of a variety of common polymers, e.g. preferably a
non-brittle polymer such as a tough elastomeric or rubbery polymer such as
nitrile rubber, butyl rubber or butyl acrylate, that is amenable to
incorporation of dispersed particles of this invention. The polymer matrix
can be ionically isolative, e.g. a butyl rubber, or ionically conducting,
e.g. a sulfonated polymer such as sulfonated polystyrene or Nafion
ionomer. The electrolyte layer is also desirably transparent or, at least,
translucent. While the ion-supplying electrolyte material can comprise a
salt dissolved in an aqueous or organic solvent-containing polymeric gel,
a preferred electrolyte material is an ionically conductive, aqueous
polymeric gel which can contain a humectant or hygroscopic filler. Useful
hygroscopic material includes deliquescent material such as lithium
chloride, calcium chloride, glycerine, sodium dihydrogen phosphate or
lithium trifluoromethyl-sulfonate. A preferred aqueous polymeric gel is
polyacrylamidomethyl-propanesulfonate, known as POLYAMPS.
In such electrochromic devices the electrochromic metal oxide material
serves as an electrode for transporting electrons into or out of the
ionically conductive electrolyte media. Concurrent with such electron
transfer is the movement of ions across an interface between said layer of
electrochromic material and said ion-supplying electrolyte layer. In
preferred embodiments of this invention the electrodes can be side by side
electrodes as disclosed in my earlier U.S. Pat. No. 5,413,739. Such side
by side electrodes are located behind, e.g. hidden by, the electrochromic
layer of the device.
In order for such electrodes to function, they must be connected to an
electrical potential by current feeders, e.g. conductive leads, which can
comprise any of a variety of conductive materials such as silver ink,
carbon ink, metal oxide ink or deposition where the metal oxide is a
conductive metal oxide such as ATO. Alternatively, the electrodes can be
in a sandwich disposition such that at least one of the electrodes should
be of transparent or translucent material to allow observation of the
electrochromic effect. Such transparent electrode material is preferably a
conductive metal oxide such as ATO. When used as a current feeder, ATO has
an optimally high conductivity in the range of 6-10% ATO. When the
transparent metal oxide electrode is used in a sandwich type display, the
current feeder is typically an integral film coating. It is believed that
ATO in a film form, as compared to the particulate doped tin oxide
materials of this invention, has such a low contrast ratio as to be
considered non-electrochromic; that is, the contrast ratio is less that
1.2.
While the following examples illustrate the preparation and use of various
embodiments of the electrochromic doped tin oxides and electrochromic
displays of this invention, it should be clear from the variety of the
examples herein that there is no intention of so limiting the scope of the
invention. On the contrary, it is intended that the breadth of the
inventions illustrated by reference to the following examples will apply
to other embodiments which would be obvious to practitioners in the
electrochromic arts.
EXAMPLE 1
This example illustrates one embodiment of an electrochromic device
according to this invention using commercially available ATO coated
TiO.sub.2 particles. 1.5 grams (g) of light grey conductive powder
comprising 12.25%ATO on 0.2 micron TiO.sub.2 particles in the weight ratio
of 23:77 from Mitsubishi Materials Company Ltd and identified as W-1
conducting particles was dispersed in 5 g of a 10% solution of
styrene-butadiene-styrene (SBS) rubber in toluene. A copper coated
polyester film was used as a electrode substrate. The dispersion was
coated as a film onto the copper layer and dried with a heat gun to
evaporate the toluene solvent. The coated substrate was immersed in an
aqueous electrolyte solution comprising 5% sodium sulfate. With the
application of 1 to 2 volts, the coating turned a visibly darker grey
color than the original color of the coating. Reversing the polarity
caused a rapid reversal to a lighter grey color that was visibly lighter
than the original color of the coating.
EXAMPLE 2
This example illustrates the fabrication of an electrochromic display
device. A first display conductor pattern was printed in the shape of a 25
millimeter (mm) square centered on a supporting substrate of polyester
film with a narrow conductor lead running from the 25 mm square to the
edge of the polyester substrate. A counter electrode conductor was printed
in the shape of a 12 mm wide line bordering the square pattern and lead at
a distance of about 1 millimeter from the edge thereof. Each conductor
pattern was printed with a conventional silver ink and coated with a
conventional carbon ink. An electrochromic display was fabricated by
overcoating the conductor pattern with a dispersion of 12% ATO-coated
TiO.sub.2 in a solution of fluorinated elastomer; the 12% ATO-coated
TiO.sub.2 was obtained from Mitsubishi Materials Company Ltd. and is
characterized as light grey conducting powder having a particle size of
0.2 micron with ATO and TiO.sub.2 present in the weight ratio of 23:77.
The fluorinated elastomer was dissolved at 22 weight percent (wt%) in
butoxyethyl acetate. Sufficient ATO coated powder was dispersed in the
elastomer solution so that the weight ratio of ATO-coated powder to
elastomer was 2.5:1. The conductor pattern was coated with the dispersion
except for the lead portions thereof at the edge of the substrate where
electrical connections could be made. The dispersion coating was dried at
130.degree. C. for 10 minutes, recoated with dispersion and redried to
provide an electrically conducting, essentially pin hole-free coating of
light grey-colored, electrochromic, doped tin oxide particles dispersed in
a transparent, ionically insulating elastomer matrix, designated as a
"basic ATO-coated display element". The electrochromic layer of the basic
ATO-coated display element was covered with a stack of adhesive polyester
gaskets to provide an electrolyte well over the electrode area; the well
was about 1 millimeter (mm) in depth and was filled with electrolyte
comprising an aqueous solution of 30 wt % lithium chloride and 5 wt %
acrylic polymer thickener, i.e. Acrysol ASE-95 from Rohm and Haas Company.
The electrolyte filled well was sealed with adhesive polyester film to
complete the construction of electrochromic display device designated D1.
EXAMPLE 3
This example illustrates the measurement of contrast ratios for an
electrochromic doped tin oxides operating in an electrochromic
effect-generating environment. The conductor leads of the electrochromic
display device D1 prepared in Example 2 were connected to a function
generator which applied a 50 millihertz, .+-.1.5 volt square wave
potential to drive the electrochromic device causing the electrochromic
ATO-coated particles visible through the electrolyte coating to cycle
between a light grey and a dark grey color as the ATO was sequentially
oxidized and reduced. The magnitude of the contrast ratio of the color
change was determined by fitting the device under a microscope fitted with
a Melles-Griot photodiode wide band width amplifier. The electrochromic,
12% ATO-containing material in the device exhibited a contrast ratio of
1.38.
EXAMPLE 4
This example illustrates the dramatic effect of an increased amount of
antimony in electrochromic properties of ATO. A basic ATO-coated display
element prepared according to Example 2 was coated with an additional
electrochromic dispersion and fabricated into an electrochromic display in
essentially the same manner of Example 2 except for the use of 33% ATO.
The contrast ratio measured in the manner of Example 3 was 1.92.
EXAMPLE 5
This example further illustrates the dramatic effect of an increased amount
of antimony in electrochromic properties of ATO. A set of basic ATO-coated
display elements prepared according to Example 2 were coated with an
additional electrochromic dispersion and fabricated into an electrochromic
display in essentially the same manner of Example 2 except that the doped
tin oxide comprised antimony in the range of 11 to 60 percent and that the
ATO and TiO.sub.2 were in the weight ratio of 36:64. The contrast ratio
measured in the manner of Example 3 is reported in Table 1.
TABLE 1
______________________________________
% Sb CR
______________________________________
11 1.6
23 2.0
33 2.14
43 2.19
47 2.17
55 1.74
60 1.94
______________________________________
EXAMPLE 6
This example illustrates the dramatic increase in electrochromic effect
achieved by selection of substrate pigment. An ATO-coated display element
prepared similar to the procedure example 2 was further coated with an
electrochromic dispersion of 12% ATO-coated onto aluminum borate (obtained
from Mitsui as Passtran 5210 conductive powder) in a fluorocarbon
elastomer solution. In the dried electrochromic top coat the weight ratio
of ATO-containing particles to fluorocarbon elastomer was 28:15. An
electrochromic device prepared as in the manner of Example 2 and evaluated
in the manner of Example 3 showed that the doped tin oxide on an aluminum
borate substrate had a contrast ratio of 5.14.
EXAMPLE 7
This example illustrates the preparation of an electrochromic,
niobium-doped tin oxide according to this invention. A barium sulfate
slurry was provided by dispersing 50 g of barium sulfate powder in 750 ml
of water and heating to 75.degree. C.; the slurry was adjusted to pH 12
with 25% sodium peroxide solution. A tin solution (98.5 g of sodium
stannate trihydrate in 250 ml of water at 75.degree. C.) was added to the
slurry. After stirring for 30 minutes, an acidic niobium solution (0.735 g
niobium trichloride in 25 ml methanol acidified with 270 cc of 20%
sulfuric acid) was added to the tin oxide-containing slurry over a 90
minute period. The pH of the niobium/tin-containing slurry was adjusted to
2.5 with 20% sulfuric acid. After 3 hours the solution was cooled and
filter washed 10 times with 250 ml of water, providing particles that were
dried in a vacuum oven at 130.degree. C. The dried particles were calcined
for 2 hours under nitrogen at 450.degree. C. to provide electrochromic
0.72% niobium-doped tin oxide coated barium sulfate substrate particles
having a contrast ratio of 1.54.
EXAMPLE 8
This example illustrates the utility of mixtures of doped tin oxide
particles and pigment particles as an electrochromic material. A mixture
of 0.4 g of 13.4% ATO particles (commercially available from DuPont as
Zelec.RTM. 301OXC ATO) and 0.15 g of TiO.sub.2 particles was dispersed in
1 g of 22 wt % fluoroelastomer solution in butoxyethyl acetate to provide
a dispersion suitable for use in an electrochromic display as described
herein. The material exhibited a contrast ratio of 2.4.
EXAMPLE 9
This example illustrates the high resistance of oxidized or reduced doped
tin oxide particles according to this invention. ZELEC 35005XC ATO
obtained from DuPont was determined to have a base ATO powder resistivity
of 0.12 ohm-cm. The base ATO was treated with sodium borohydride, washed
and dried to provide reduced ATO (having 0.58% sodium ions) having a
powder resistivity of 404 ohm-cm. The base ATO was treated with ammonium
persulfate to provide oxidized ATO having a powder resistivity of 5.5
ohm-cm. When the oxidized ATO is heated, the powder resistivity returns to
a value close to 0.1 ohm-cm.
While specific embodiments have been described herein, it should be
apparent to those skilled in the art that various modifications thereof
can be made without departing from the true spirit and scope of the
invention. Accordingly, it is intended that the following claims cover all
such modifications within the full inventive concept.
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